Is there a way to have the main desktop appear on my external monitor and span onto the PowerBook screen instead of vice versa?

Not 100%for powerbok but on my iBook
System Prefs>Displays>Arrrangment . Just drag the screens around in to the positions you want them. Move the white bar at the top to the screen you want to be the primary.

If you position the little screen so its edges aren't lined up with the edges of the larger screen then expose will still work on the larger screen.